In respect to image compression, there is disclosed a thinning-out method for thinning out chrominance element data by utilizing the characteristic of human eyes that the human eyes are relatively insensitive to the change of color, by Japanese Patent Laying-Open No. 7-221993(1995). According to this thinning-out method, the rate of change of chrominance elements in a small block is obtained, the chrominance elements are slightly thinned out if the rate of change is high and the chrominance elements are largely thinned out if the rate of change is low, thereby compressing image data.
Generally, however, if chrominance elements are thinned out, there often occurs a phenomenon referred to as ‘color deviation phenomenon’ that false colors are generated. Such a color deviation phenomenon is particularly conspicuous on edges of a character.
Also, according to a conventional thinning-out method, a thinning-out distance is determined based only on the rate of change of chrominance elements. Due to this, a thinning-out processing is conducted at the same thinning-out distance to a small block consisting of a character having a single color as shown in FIG. 20A and a small block consisting of a graphic form having a plurality of colors as shown in FIG. 20B. If many colors are mixed as shown in FIG. 20B, a person cannot grasp the graphic form in the small block as an object such as a character, with the result that even if color deviation somewhat occurs, the person tends not to notice it. Due to this, in such a case, it must be possible to increase the thinning-out distance.